According to me, a good business idea is one that gathers all these conditions:

- it isn't based on any short term trend
- it doesn't need to target multiple markets
- it is something that will be purchased many times by the same customer ( repeat business )
- it needs little work/labour to be operated
- it can be advertised easily by words of mouth ( wow factor )
- it needs almost zero investment
- it can be operated from almost any location, any country.
- it doesn't rely on anyone else than you
- it's not season-dependant
- it's legal ( obvious )

It would be difficult to find a business that requires zero investment. Although of course lower the investment, the less the risk. You can also get a good return on investment if you acquire law firms for sale.